Homoallylic sterols

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

Loh et al. found a tritlic acid-catalyzed 2-oxonia Cope rearrangement, which was used in the stereocontrolled synthesis of linear 22i -homoallylic sterols (eq 48). Interestingly, poor stereoselectivity was observed when In(OTf)3 was employed as the catalyst for this reaction. Stereoselective Mannich-type reaction of chiral aldimines with 2-silyloxybutadienes in the presence of triflic acid gives the corresponding products with 70-92% de in 62-74% chemical yield, which are not obtained by general Lewis acid-promoted methods (eq 49). ... 

The Loh group has reported a homoallylic sterol/indium(III) Lewis acid reagent system for the enantioselective and a-regioselective allylation of aldehydes. Allyl transfer from y-adduct homoallylic sterol to benzaldehyde furnishes the a-adduct in good yield, excellent ee, and E-conflguration of the double bond (eq 14). 

The aluminium alkoxides present in the Oppenauer oxidation can cause some base-induced side reactions. Thus, quite typically during the oxidation of sterols possessing homoallylic alcohols, a migration of the alkene into conjugation with the resulting ketone is observed (see pages 256 and 259).4... 

Van Tamelen showcased the ability of homoallylic alcohols to bias the stereochemical course of epoxidations in investigations of the biosynthesis of sterol precursors (Equation 11) [68]. The polyunsaturated substrate 39 readily participated in a vanadium-catalyzed epoxidation of the 1,1-disubsti-tuted olefin with complete regio- and stereoselectivity. Monoepoxide 40 was isolated as the only detectable product in quantitative yield. 

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